Cellular communication systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones allow users to place and receive phone calls most anywhere they travel. Moreover, as cellular telephone technology is advanced, so too has the functionality of cellular devices. For example, many cellular devices now incorporate Personal Digital Assistant (PDA) features such as calendars, address books, task lists, calculators, memo and writing programs, etc. These multi-function devices usually allow users to wirelessly send and receive electronic mail (email) messages and access the Internet via a cellular network and/or a wireless local area network (WLAN), for example.
Cellular devices have radio frequency (RF) processing circuits and receive or transmit radio communications signals typically using modulation schemes. Constant envelope signals use phase modulation to represent/encode information; however, their amplitude does not change with time. In contrast, non-constant envelope modulation schemes encode information in amplitude and phase and are typically generated using quadrature transmit paths (I/Q paths). There are several amplitude modulation schemes, such as 8 phase-shift keying (8PSK) used in second generation cellular transceivers, quadrature phase-shift keying (QPSK) used in third generation cellular transceivers, and orthogonal frequency-division multiplexing (OFDM) used in fourth generation cellular transceivers, all typically generated using a quadrature transmitter. In contrast to constant envelope modulation, quadrature modulation and demodulation circuits may create linearity issues with power amplifiers because the peak power transmitted is higher than average power, and therefore the PA is mostly operated in the “backed-off” condition, where it is inefficient. This drawback may be further exacerbated under the condition of poor antenna match. This can cause some degradation of total radiated power (TRP) and raise harmonic interference issues because of the greater non-linearity of a power amplifier.
In particular, cellular devices that use Quadrature modulations circuits may experience difficulty in transmitting large bandwidth signals, for example, third and fourth generation cellular transceiver signals. In particular, the large bandwidth of these signals may demand a fairly linear amplifier, which may prove to be quite power inefficient, thereby hurting battery life.